The present invention relates to a battery including an organic electrolyte (organic electrolyte battery) used for primary or memory back-up power sources of electronic appliances. More particularly, the present invention relates to a coin-shaped organic electrolyte battery with thermal resistance at a high temperature which can be mounted onto a circuit substrate by automatic soldering according to the Reflow method.
Organic electrolyte batteries generally have a high energy density so that it is possible to make the electronic appliances compact and light. Also, they have superior reliability in terms of storage characteristics and leakage resistance so that there is an increasing demand for them as primary and memory back-up power sources for various electronic appliances. Majority of this type of batteries are unchargeable primary batteries. Their representative experiment is batteries using metallic lithium as a negative electrode, and manganese dioxide, carbon fluoride, thionyl chloride, sulfur dioxide or silver chromate as a positive electrode.
Recently, rechargeable secondary batteries have been developed, and particularly, coin-shaped lithium secondary batteries using a lithium-aluminum alloy or the like have been in practical use for several years. Among these batteries, those using vanadium pentoxide or lithium manganate as a positive electrode are generally used.
A common organic electrolyte of such secondary batteries is one obtained by dissolving a lithium salt as a solute in a mixture solvent which contains a solvent having a high boiling point and a high dielectric constant and a solvent having a low boiling point and a low viscosity. For example, one or more solvents such as ethylene carbonate, propylene carbonate, butylene carbonate and .gamma.-butylolactone are used as the solvent having a high boiling point and a high dielectric constant. The low-viscosity solvents mixed for reducing viscosity and thereby enhancing conductivity are intended to facilitate movement of lithium ions and ensure smooth discharge reaction of the batteries. For example, one or more solvents such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran and 1,3-dioxorane are used for this purpose. As the solute, lithium salts such as LiClO.sub.4, LiBF.sub.4, LiPF.sub.6 are generally known.
However, the batteries including the above-described organic electrolyte have various problems, if left under a high temperature. For example, the organic electrolyte evaporates at a high temperature in the battery to be obtained. Especially, the low boiling point solvents in the mixture solvent, which is retained in a separator, evaporate due to the boiling point of as low as around 100.degree. C. Also, the above-mentioned lithium salts thermally decompose at a high temperature, thereby losing their function, since any of these lithium salts has a thermal decomposition temperature of around 100.degree. C. This means occurrence of a trouble which promotes deterioration of battery performance. Therefore, the organic electrolyte batteries have a limit of the temperature at which they can be used, with their upper limit set at 60 to 85.degree. C.
Under these circumstances, vigorous development is recently in progress on an extremely compact, coin-shaped secondary batteries having a diameter of not more than 6 mm to serve as memory back-up power sources for small-sized portable appliances. In order to mount such extremely compact batteries onto print substrates, there has been a proposed method for mounting lead terminals of the batteries by an automatic soldering using the Reflow method. According to this proposal, however, the internal temperature of the Reflow furnace becomes high, although for a short time, and reaches as high as 250.degree. C. for dozens of seconds at the peak. Therefore, as described above, if the batteries of normal configuration are caused to pass the Reflow furnace, the oroganic electrolyte instantly vaporizes to raise internal pressure of the batteries, which may result in explosion of the batteries themselves.
In addition, it is also important whether each component of the organic electrolyte batteries has sufficient thermal resistance. Generally, a gasket insulating a positive can and a negative can (a seal plate) and a separator insulating a positive electrode and a negative electrode are made of polypropylene. Since thermosoftening temperature of polypropylene is 100 to 120.degree. C., the gasket and the separator are damaged by heat, when they are exposed to a much higher temperature than the thermosoftening temperature in passing the Reflow furnace.
In order to solve the problems of the batteries induced by such high temperature environment, there has been another proposal of the organic electrolyte batteries wherein battery components are conferred thermal resistance (for example, Japanese Laid-open Patent Publication Hei 8-321287). The organic electrolyte batteries according to this proposal comprises an organic electrolyte obtained by dissolving a lithium salt as a solute in an organic solvent having a boiling point of not less than 170.degree. C., a separator of porous synthetic resin sheet having a boiling point of not less than 170.degree. C. and a gasket of thermoplastic synthetic resin which can be continuously used at least at 150.degree. C.
More particularly, the proposed batteries use an organic electrolyte comprising lithium borofluoride dissolved as a solute in a solvent containing .gamma.-butylolactone, a separator and a gasket made of heat resistant resin such as polyphenylene sulfide.
However, the proposed organic electrolyte batteries are intended to be used and stored in an environment of more than 150.degree. C. for a long period, thereby not having enough thermal resistance to withstand the temperature of not less than 250.degree. C. required for the Reflow method. Therefore, they also have the same problems as conventional other batteries such as the acute vaporization of the organic solvents, the decomposition of the solute and the damage of the gasket and the separator.
As described above, the currently available organic electrolyte batteries do not have enough thermal resistance to endure 250.degree. C. of the Reflow furnace. As the result, the organic electrolyte batteries cannot yet be mounted onto a circuit substrate by the automatic soldering according to the Reflow method.
Therefore, the object of the present invention is to provide an organ ic electrolyte battery having a high thermal resistance at a high temperature that has never accomplished before by combining solvents with lithium salts, both having thermal resistance and reliability.
Further, the object of the present invention is to provide an organic electrolyte battery having an excellent thermal resistance that can endure the temperature of about 250.degree. C. required for the automatic soldering according to the Reflow method, by employing highly heat resistant materials compatible with an organic electrolyte for battery components such as a gasket and a separator.